Duplex vacuum-tube repeater



ATTOR N EY N. P. CASE DUPLEX VACUUM-TUBE REPEATER Filed July 13, 1938 May 2l, 1940.

Patented May 21, 1940 J UNITED STATES PATENT OFFICE DUPLEX VACUUM-TUBE REPEATER ,N Nelson P. Case, Great Neck, N. Y., assgnor to Hazeltine Corporation, a corporation of Dela- Ware Application July 13, 1938, Serial No. 218,937

i 1. 4 Claims.

This invention relates generally to a duplex amplifier or repeater stage and particularly to Such a stage comprising two output circuits, one

, of which is substantially free from frequency 5 components of one of the signals being amplified or repeated, and is also substantially free from heterodyne sum or difference frequency components of signals being amplified or repeated.

Vacuum-tube repeaters are commonly utilized to repeat or amplify two or more signal inputs having components of substantially different frequencies. Such devices have been utilized to provide a reflex amplification of a signal in a modulated-carrier signal receiver, thus to minimize the number of tubes required to provide a desired gain. Such refiex amplifier stages have been utilized to effect a dual amplification of an audio-frequency signal and a carrier-frequency signal or, in the case of a superheterodyne receiver, to effect an amplification of an intermediate-frequency signal and a carrier-frequency signal or an audio-frequency signal. Such stages of the prior art, however, have had 25 the disadvantage that frequency components of each of the signals amplified, as well as heterodyne sum and difference frequency components of the signals amplified, were present Ain the output circuit or output circuits of the stage. It has, therefore, been essential in the prior art systems that all components of each of the signals to be repeated or amplified `be of a substantially different frequency than y all other signal or modulation components, that is, of a sufficiently different frequency so that they can be separated by filters. If the above conditions are not satisfied, it has been impossible to separate the repeated or amplified signals into separate useful channels in the cusi0 tomary manner. These conditions have necessarily placed restrictions on the relationship of frequencies of the signal components which may be successfully amplified in the same stage.

A Also, since it is impossible to construct a theoret- '315 ically perfect filter, the operation of all duplex amplifier stages of the prior art has been impaired to some degree.

It is an object of the invention to provide an improved duplex repeater stage.

v'50 It is another object of the invention to provide a duplex repeater stage having an output circuit in which are developed only signal components of the frequency of one of the signals being repeated by the stage. 55 It is a further object ofthe invention to provide a duplex repeater in which only the components of one of the signals being repeated need be of a substantially different frequency than the sum or difference frequency components of the two signals being repeated. 5

In accordance with the invention there is provided a duplex repeater stage comprising a vacuum tube including two repeater sections utilizing the same electron stream derived from the cathode of the tube. The rst section in- 10 cludes the cathode, a first signal-input electrode and a rst output electrode, while the second section includes a second signal-input electrode and a second output' electrode, in the order named, the first output electrode having portions 15 respectively disposed between the cathode and the second signal-input electrode and between the second signal-input electrode and the second output electrode. The stage includes provisions, such as the spacing of the electrode elements, 420 for substantially eliminating the transconductance from the second input electrode to the first output electrode. A circuit is provided for applying a signal comprising frequencies within a predetermined range to the second signall25 input electrode and another circuit is provided for applying a signal comprising frequencies within a higher range than the first-mentioned vrange to the first signal-input electrode, the fre- `an amplification of both the radio-frequency signal and the intermediate-frequency signal of the receiver.

In general, the receiver illustrated in the drawing comprises a tunable radio-frequency selector circuit, comprising an inductance IIJ and a condenser II coupled between an antenna-ground i circuit, comprising an inductance I2 and a radiofrequency signal-input grid of a duplex amplifier stage comprising tube I3. The radio-frequency output electrode of tube I3 is coupled through an inductance I4 to a second tunable radio-fre- ,55

T'IIS quency selector circuit comprising an inductance I5 and a condenser IS. The selector I5, I5 is, in turn, connected to a frequency-changer tube I'I comprising an oscillator section including the frequency-determining circuit I8, I S. The condensers II, It, and I9 are provided with a conventional unicontrol mechanism shown schematically by the `broken lines interconnecting these elements. The output electrode cf frequency changer Il is coupled through a double-tuned intermediate-frequency transformer 2t to a second signal-input grid of tube i3. The amplied intermediate-frequency output is taken from the anode or second output electrode circuit of tube I3, which is coupied to the input circuit of an intermediate-frequency amplifier 22. Connected in cascade to the output circuit of .intermediatefrequency amplifier 22, in the order named, are a signal detector and automatic amplification control or A. V. C. rectifier 23, an audio-frequency amplifier 2, and a loud-speaker 25. An A. V. C. bias derived from unit 23 is applied to the control electrodes of one or more of the tubes I3, il and'intermediate-frequency amplifier 22 by way of aconnection 2l and suitable filter circuits.

In general, the operation of the receiver just described, neglecting the features which constitute the present invention presently to be described, is well understood in the art and a detailed description thereof is, therefore, unnecessary. In brief, however, signals intercepted by the antenna circuit comprising inductance I2 are selected in selector circuit lil, II, amplified in the inner ror radio-frequency amplifier portion of Vacuum tube I3, and transmitted by way of selector circuit I5, I 6 to frequency changer i1, wherein they are converted to intermediatefrequency signals in a well-known manner. The intermediate-frequency signals are further selected in the selector circuit comprising doubletuned transformer 20 and are amplified by a reflex amplification in the outer or intermediatefrequency amplifier section of tube I3 and passed to intermediate-frequency amplier 22 wherein they are further amplified. Audio frequencies of modulation are derived in detector 23, are further amplified in audio-frequency amplifier 2li, and supplied in the usual manner to loudspeaker 25 for reproduction. The undirectional-bias voltage developed by the A. V. C. rectifier and detector 23 is lapplied by way of the connection 2l to both input electrodes of tube i3, tube I7, and one or more of the tubes of intermediatefrequency amplifier 22, thereby maintaining the amplitude of the signal input to detector 23 within a relatively narrow range for a wide range of received signal amplitudes.

Referring now more particularly to the parts of the system involving the present invention, it is seen that the inner or radio-frequency aniplifier section of vacuum tube I3 includes the first grid as a signal-input electrode and, as an youtput electrode, the second and the fourth grids of the tube connected together within the tube.

The outer or intermediate-frequency amplifier section of tube I3 includes the anode or second output electrode of the| tube as an output electrode, the third grid as a signal-input electrode, and a virtual cathode formedbetween the second and third grids. A cathode-biasing resistor 33, by-passed for alternating currents by condenser 30', is provided for vacuum tube I3, while suit- V`able operatingr potentials are supplied for the system through the terminals marked B1 and B2, the output electrode comprising the second and fourth grids of tube I3 being supplied through a network comprising series resistors 3|, 32 and shunt condenser 33. The first and third grids of tube I3 which are negatively biased with respect to the cathode, together with the second grid, which is maintained at a positive potential by the source B2, are effective to form a virtual cathode in the tube between the second and third grids ina manner which -is well understood. Tube I3 is so designed that transconductance between the third grid and the second and fourth grids together is substantially eliminated over the radio-frequency range of the receiver shown. It has been found that a type 6L? tube meets this requirement fairly well.

In considering the operation of the duplex amplifier stage 'comprising tube I3, `it will be seen that, Vbecause of the arrangement of the electrodes within the tube and the potentials on the second and third grids with respect to the actual cathode, a virtual cathode, that is, a cloud or mass of slowly or random moving electrons, is formed between the second and third grids. The flow of electrons to the anode 'or second output electrode of the tube from this virtual cathode is controlled by the third grid. As the potential of the third grid of the tube is varied, the electron current to the secondgrid, which is connected to the output circuit including inductance I4, varies in a manner opposite to that to the fourth grid, which is also connected to the output circuit including inductance I4. The second and the fourth grids may, therefore, be so related to the third grid'that transconductance between the third grid and the composite output electrode comprising the second and fourth grids together is substantially eliminated over the range of frequencies to which the selectors I E, II and I5, I6 are tunable. In other Words, the second and fourth grids may be said to be a means for eliminating transconductance between the outer signal-input electrode or third grid and the inner output electrode. The automatic volume control bias applied to the first grid is effective to control the amplification of radio-frequency signals as well as intermediate-frequencysignals within the tube I3, while that applied to the third grid or second control electrode is effective only to control the' amplification of the intermediate-frequency signals. Either may be used separately if desired.

While there has been shown a duplex amplifier in which a radio-frequency signal is amplified in the inner section and an intermediate-frequency signal is amplified in the outer section, it will be understood that the invention can be used as well to amplify a carrier-frequency signal in the inner section and to amplify an audiofrequency signal in the outer section thereof. The output circuit comprising inductance I4 is preferably designed to have a low impedance over the range of frequencies amplified in the outer section of the duplex amplifier in order to prevent the development ofany signal voltages within such range in case of any incidental transconductance between the second signalinput grid and rthe first output electrode. The output circuit of the intermediate-frequency amplifier section of tube I3, that is, the circuit comprising the input circuit of intermediate-frequency amplifier 22, is preferably designed to have a low impedance to signals within the radiofrequency range of the receiver. ItA will also be understood that the A. V. C. potential applied :to the first grid of tube I3 is effective to control yelli() the amplication of each of the signals being amplified within the tube. The duplex amplifier stage of the invention is preferably utilized With the inner section acting as an amplifier of the higher frequency signals in order to prevent a modulation of the high-frequency signal by signal components of the low-frequency signal.

While the invention is of general application, there follow specications of a particular duplex amplier stage having amplification characteristics which are particularly satisfactory:

Tube 13 Type 6L? Tube 17 Type 6A8 Resistor 30 1,000 ohms Condenser 30 0.05 microfarad Resistors 31 and 32 10,000 ohms Condenser 33 8 microfarads Source B1 105 volts Source B2 230 volts `Radio-frequency selecto-rs I0, Il

and l5, I6 tunable over a range of frequencies from 0.54 to 1.56 megacycles.

Double-tuned transformer 20 tuned to an intermediatefrequency of 455 kilocycles.

While there has been described What is at present considered to be the preferred embodiment of this invention, it Will be obvious to those skilled in the art that Various changes and modifications may be made therein Without departing from the invention, and it. is, therefore, aimed v in the appended claims to cover all such changes andmodications as fall Within the true spirit and scope of the invention.

What is claimed is:

1. A duplex repeater stage comprising a vacuum tube including a rst repeater section having a cathode, a rst signal-input electrode and a first output electrode, a second repeater section comprising a second signal-input electrode and a second output electrode in the order named, said rst output electrode having portions respectively disposed between said cathode and said second signal-input electrode and between said second signal-input electrode and said second output electrode, said portions being spaced with relation to said second signal-input electrode so that transconductance from said second signal-input electrode to said first output electrode is substantially eliminated, a circuit for applying a signal comprising frequencies within a predetermined range to said second signal-input electrode, and a circuit for applying a signal comprising frequencies Within a range higher than said iirstmentioned range to said first signal-input electrode, the frequencies of said first-mentioned range being substantially diiferent from' the frequencies of said higher range or the heterodyne sum or difference frequencies of said ranges.

2. In a modulated-carrier signal receiver of the superheterodyne type, a duplex repeater stage comprising a vacuum tube including a rstrepeater section having a cathode, a first signalinput electrode and a rst output electrode, means for forming a virtual cathode, a second repeater section comprising said virtual cathode, a second signal-input electrode and a second output electrode disposed in the order named, means for substantially eliminating transconductance from said second signal-input electrode to said iirst output electrode over the intermediate-frequency range of said receiver, a circuit for applying intermediate-frequency signals to said second signal-input electrode, a circuit for applying radiofrequency signals to said rst signal-input electrode, the frequencies of said intermediate-frequency signal being of a substantially different magnitude than the frequencies of said radiofrequency signal or the heterodyne sum or difference frequencies of said signals, and means for effecting an automatic amplication control of each of said signals Within said tube.

3. In a modulated-carrier signal receiver of the superheterodyne type, a duplex repeater stage comprising a vacuum tube including a rst repeater section having a cathode, a first signalinput electrode and a first output electrode, means for forming a virtual cathode, a second repeater section comprising said virtual cathode, a second signal-input electrode and a second output electrode disposed in the order named, means for substantially eliminating transconductance from said second signal-input electrode to said rst output electrode, a circuit for applying intermediate-frequency signals to said second signalinput electrode, a circuit for applying radio-frequency signals to said first signal-input electrode, the frequencies of said intermediate-frequency signal being of a substantially different magnie tude than the frequencies of said radio-frequency signal or the heterodyne sum o-r difference frequencies of said signals, and means for effecting an automatic volume control of each cf said signals Within said tube by a single bias applied to one of said electrodes.

4. In a modulated-carrier signal receiver of the superheterodyne type, a duplex repeater stage comprising a vacuum tube including a rst repeater section having a cathode, a first signalinput electrode and a first output electrode, means for forming a virtual cathode, a second repeater section comprising said virtual cathode, a second signal-input electrode and a second output electrode disposed in the order named, means for substantially eliminating transconductance from said second signal-input electrode to said rst output electrode, a circuit for applying intermediate-frequency signals to said second signalinput electrode, a circuit for applying radio-frequency signals to said iirst signal-input electrode, the frequencies of said intermediate-frequency signal being of a substantially different magnitude than the frequencies of said radio-frequency signal cr the heterodyne sum or difference frequencies of said signals, and means for effecting an automatic amplification control of only said intermediate-frequency signals Within said tube by a bias applied to one of said electrodes.

NELSON P. CAsE. 

